This invention relates to a process of coking lump fuel at high temperatures in shaft ovens which are heated only with hot scavenging gases which have been produced in a combustion chamber and are conducted through the coking zone in a counter-current to the lump fuel.
The degasification of fuel with scavenging gases has found a wide application in the production of low-temperature tar. This dry distillation using a scavenging gas is carried out at low or medium temperatures of about 450.degree. - 650.degree. C. and results in a high yield of tar and in a coke having a relatively high content of volatile constituents. The hot scavenging gas is produced from the gas produced by dry distillation in that part of the latter gas is burned when tar and water have been removed by condensation. By this treatment, the CO.sub.2 and water vapor contents of the scavenging gas are increased.
Difficulties are involved in this operation if the coking temperature is to be increased above 750.degree. - 800.degree. C. At these temperatures, carbon dioxide and water vapor react with solid carbon to form CO and H.sub.2. These known endothermic gasification reactions take place at such a high rate that the temperature of the lump of coke is appreciably reduced. As a result, there is a difference between the temperature of the scavenging gases entering the coking zone and the final temperature of the coking treatment. The decrease of the temperature of the scavenging gas is due to the heat requirement of the undesired endothermic gasification reactions.
An increase of the inlet temperature of the scavenging gas intensifies mainly the endothermic gasification reactions and has only a very small influence on the coking temperature.
The degree of decomposition of the gasifying agents depends not only on the operating conditions, particularly on the temperature of the scavenging gas, but also on the reactivity of the solid fuel. An indirect measure of this influence is the residual content of volatile constituents remaining in the coke. For instance, if lignite briquettes are coked to form a highly reactive coke, the content of volatile constituents in the coke can hardly be decreased below 4% if scavenging gases are used which contain CO.sub.2 and H.sub.2 O. On the other hand, the content of volatile constituents may well be decreased to 1.2 - 1.5% in coke which has a low reactivity, such as is produced, e.g., by the degasification of anthracite briquettes. As regards the final temperature of the coking process, the gasification reactions might be tolerated, particularly with fuels having a low reactivity, but these reactions result in two additional serious disadvantages. One of these is the decrease of the yield of coke by 1 - 6%, depending on the operating conditions. The other disadvantage is more significant and resides in the weakening of the coke structure.
These gasification reactions do not only take place on the external surface of the lump of fuel but also on its internal surfaces so that not only the abrasion resistance of the coke but its mechanical strength as a whole is reduced. This results, e.g., in a lower crushing strength and in lower stability and hardness values according to ASTM. The gasification results in such a deterioration of the coke that the product of the process may no longer be satisfactory if a high-strength coke is required. For this reason, the coking with scavenging gases at high temperature has not been successful so far in spite of its numerous advantages.
To avoid the disadvantages, it has already been proposed to heat the scavenging gases not directly by a partial combustion but indirectly in recuperators. In this way, the combustion products may be kept from the scavenging gas. Owing to the high temperatures, however, the recuperators become very expensive. Besides, cracked products deposited on the pipe surfaces give rise to trouble in operation.
In another process, the scavenging gases may be heated in regenerators heated in alternation. This concept has proved to be practicable in pilot plant operations but is too expensive and for this reason has not been used in practice.
DOS 1,471,588 describes a process of continuously coking fine-grained coal in a gas stream. In that process, the fine-grained coal is centrally and axially blown into a cylindrical degasification chamber and a highly preheated carrier gas is blown in tangentially to the coal at the roof of the jet of fine-grained coal. That carrier gas is a hot combustion gas, which after an addition of steam is subjected to a water gas reaction in a chamber which is filled with lump coke so that also surplus oxygen is consumed. The presence of CO.sub.2 in the carrier gas is not taken into account. The process is intended for use in the coking of fine-grained fuels.
German Pat. No. 369,885 describes a process of improving the calorific value and of increasing the yield of a gas produced by dry distillation. Oil is injected into the hot scavenging gases between the combustion chamber and the kiln. This carbonization results at the same time in a reduction of the scavenging gas temperature. Because that process is intended for the low temperatur carbonisation of fuels, the above-described gasification reactions have not been taken into account because they may be neglected in the low temperature range.